The Effect Of Rare Element Gd And Y On The Icosahedral Quasicrystal,Microstructure And Properties Of Mg-Zn-(GdY) Alloy

Mg-Zn-RE（RE=Gd,Y）alloy in this thesis is prepared by high frequency induction melting method by controlling the composition of each elements and heating treatment.The quasicrystal alloy with different content was prepared to investigate the effect of rare element Gd and Y to I-phase’s evolution of microstructure and composition.The mainly characterization technique is optical microscopy（OM）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）.This thesis mainly includes three parts:（1）The observation of I-phase’s microstructure in Mg-Zn-Gd alloy and Mg-Zn-Gd-Y alloy;（2）The use of element Y and Gd in Mg-Zn-RE alloy in the process of I-phase’s formation,and the summary of I-phase’s structure rule formation in Mg-Zn-RE alloy;（3）The influence on Mg-Zn-Gd-Y alloy’s hardness and I-phase’s microstructure after solid solution and aging heating treatment.First of all,Mg-Zn-Gd alloy and Mg-Zn-Gd-Y alloy is prepared in high frequency induction melting apparatus according to the corresponding composition design.The scanning electron microscopy（SEM）is used to observe the microstructure of I-phase.Combined with the analysis result of energy spectrum analysis（EDS）and X-ray diffraction（XRD）,I-phase does exist in Mg-Zn-Gd and Mg-Zn-Gd-Y alloy.Fivefold rotational symmetry structure and primary icosahedral quasicrystalline phase is verified by using transmission electron microscopy（TEM）and selected area electron diffraction（SAED）.Secondly,the formation rule of I-phase in Mg-Zn-Gd-Y quaternary alloy is investigated.In the nominal composition of（MgZn）₈₅Gd_（15-z）Y_z（2.5<Z<5.5）quaternary alloy,the content of Gd plays a main role for the growth of the stability of I-phase.In I-phase,when the ratio of Gd is higher than 3.94%,the growth completion of I-phase gets better as the ratio of Gd ascend.The completion achieves the highest when the ratio is 5.65%,after that,the number of Gd content hasn’t a further effect to the growth of I-phase;Y element is used to refine grain in Mg-Zn-Gd-Y alloy,when the ratio of Y is 0.21%≤Y≤2.90%,the refines of grain gets better as the ratio of Y ascend.After that,I-phase in different growth stage is prepared by controlled cooling.The experimental result shows that,I-phase separate out from liquid phase by peritectic reaction of L+（Mg,Zn）₅RE =I-phase,the morphological structure of Mg-Zn-Gd alloy is pataloid structure or polygon structure,while the morphological structure of Mg-Zn-Gd-Y alloy is pataloid structure or chain structure.The experimental result after solid solution aging heat treatment shows that,solid solution treatment does can refine the grain of Mg-Zn-Gd-Y alloy effectively,and the hardness of Mg-Zn-Gd-Y alloy enhance better after aging treatment.